In a fuel cell power plant chemical energy contained in the feed gas is converted into electrical energy by electrochemical reactions in the fuel cell. The feed gas is electrochemically oxidized at the anode of the fuel cell to give up electrons, which are combined with oxidant reactant gas in the cathode of the cell. Typical fuel cells widely employed in the known fuel cell power plants operates on hydrogen-carbon oxide fuel and air oxidant. Such a fuel cell is the known molten carbonate fuel cell and the solid oxide fuel cell, wherein H.sub.2 -O.sub.2 -fuel contained in the feed gas and the cathode oxidant gas is converted into water by the electrochemical reactions.
When using a molten carbonate fuel cell CO.sub.2 is required in the cathode to maintain an appropriate ion transport through an electrolyte matrix, which is in contact with the anode and the cathode. CO.sub.2 is produced at the anode according to the following reactions: EQU H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e (1) EQU CO+CO.sub.3.sup.2- .fwdarw.2 CO.sub.2 +2e.sup.- ( 2)
and consumed at the cathode by the reaction: EQU 1/2O.sub.2 +CO.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.2- ( 3)
The theoretical thermal efficiency of a H.sub.2 -O.sub.2 fuel cell is determined by the ratio of free energy and heat of reaction of the overall cell reaction. EQU H.sub.2 +1/2O.sub.2 .fwdarw.H.sub.2 O; .DELTA.H=-241,8 kJ/mole(4)
Though the free energy of the oxidation of H.sub.2 and CO decreases with an increase in temperature, giving a decreased reversible voltage of the cell, the performance of a practical fuel cell is kinetically controlled and benefits from a temperature increase. Mechanical properties and constraints of the materials used in the cell components, however, limit the operating temperature of the cells to a rather narrow temperature interval in order to avoid structural stress of the electrode material or electrolyte degradation, caused by sintering or crystallization of the electrolyte matrix. The operating temperature of e.g. a conventional molten carbonate fuel cell is confined within the range of 600.degree.-700.degree. C. Thus surplus of heat generated by the exothermic electrochemical processes and polarization loss in the cell has to be removed from the cell.
Cooling of the fuel cells, which in a fuel cell power plant are piled up to a stack of many individual cells, is provided by heat exchanging plates, or channels with a stream of a coolant to keep the stack at its optimum operating temperature. In the known fuel cell power plants cathode oxidant gas is used as coolant in the stack. Hot cathode exhaust gas is cooled and mixed with air along with CO.sub.2 from anode exhaust gas before it is recycled to the cathode compartment. The flow of the mixed recycle gas is adjusted at an appropriate rate depending on the cooling demand of the fuel cell stack in order to provide sufficient cooling.
A drawback of the known fuel cell power plants using coal gas or hydrogen-carbon oxide rich gases as feed is the high cooling demand of the fuel cell stack caused by the strongly exothermic conversion reactions in the fuel cells.
In particular, the recycle and gas supply system of the cathode gas loop has to be designed for a high gas flow to meet the cooling demand resulting in piping with considerable sectional areas and large compression units with high energy consumption for providing a sufficient gas flow.
It is an object of the present invention to provide a fuel cell power plant employing hydrogen and carbon oxide rich gases as feed and having an improved overall efficiency by reducing the demand of cooling the fuel cell of the plant.
It is further an object of the present invention to simplify the cathode gas supply loop of such a fuel cell power plant.